This invention relates to a vapor reflow type soldering apparatus and more particularly to a vapor reflow type soldering apparatus suitable for soldering I.C. packages, resistors, capacitors, etc. to a printed circuit board.
Vapor reflow type soldering apparatus suitable for soldering I.C. packages, resistors, capacitors, etc. to a printed circuit board are disclosed in U.S. Pat. Nos. 4,735,001, 4,776,105 and 4,809,443. In these apparatus, a printed circuit board with electronic parts mounted thereon is first preheated in the preheating chamber provided near the inlet of the transfer passage and then, it is transferred into the vapor generating tank in which saturated vapor of a thermal medium is contained. In the vapor generating tank, a part of the saturated vapor is discharged from the longitudinal vapor discharge openings which are opened along the transfer passage and it is supplied on the printed circuit board. The saturated vapor concentrates at the leads of the electronic pats to be soldered, and the leads, particularly, are heated by the condensation latent heat of the saturated vapor. By this heating, the solder paste placed at the positions corresponding to the leads is melted. Then, the printed circuit board is transferred out of the vapor generating tank into the cooling chamber. By the final step of cooling, the solder solidifies and the soldering process is thus completed.
The conventional vapor reflow type soldering apparatus as described above have the following problems.
(1) Between the preheating chamber and the vapor generating tank, there is provided beneath the transfer passage a cooler for cooling and collecting the vapor that has flowed into the transfer passage. As a result, while transferred from the preheating chamber through the transfer passage to the vapor generating tank, the printed circuit board is cooled, thus reducing the preheating effects. Further, in soldering, the solder can increase in strength if it is cooled quickly after melted. Therefore, the printed circuit board that comes out of the vapor generating tank should preferably be cooled quickly. Another cooler is provided beneath the transfer passage between the vapor generating tank and the cooling chamber in order to cool and collect the vapor that has flowed into the transfer passage. This cooler is capable of quick, cooling the printed circuit board. However, a certain distance set between this cooler and the cooling chamber so as to prevent the flow of the vapor out of the apparatus contributes to slackening the cooling speed of the printed circuit board. Consequently, there is uneasiness about the reliability of soldering.
(2) The density of a thermal medium used for heating the printed circuit board is as high as 2-0 times that of air. Therefore, the vapor falls straight downward immediately after discharged from the longitudinal vapor discharge openings. Then, the vapor flows width-wise. With printed circuit boards having electronic parts mounted thereon at high density, it is difficult for the vapor discharged from the longitudinal vapor discharge openings to reach the central area of the printed circuit board as its flow is obstructed by the electronic parts mounted thereon. As a result, the temperature distribution across the width of the printed circuit board becomes uneven. Further, in a case where the width of the conveyor constituting the transfer passage is varied in accordance with the width of the printed circuit board, the vapor discharged from a longitudinal vapor discharge opening located at a movable conveyor guide, which has been moved to vary the width, is hindered by the conveyor guide from flowing toward the printed circuit board, so that the amount of the vapor reaching the printed circuit board becomes small in comparison with the vapor discharged from a longitudinal vapor discharge opening located at the fixed conveyor guide. Also in this case, the temperature distribution across the width of the printed circuit board becomes uneven, so that the reliability of the soldering is lowered.